Evaluation of the radiative recombination mechanism in Si nanocrystals embedded in silica matrix

We report measurements of the temperature dependence of photoluminescence (PL) life-time and efficiency of Si nanocrystals (Si-Nc) embedded in silica matrix. We use a practical technique based on lock-in acquisition that allows us to simultaneously evaluate, at each emission-energy, intensity and decay-time of the detected signal. Samples are prepared by Silicon-ion implantation in a SiO2 layer followed by thermal annealing. The implantation dose of Si ions ranges between 2 × 1016 cm-2 and 2 × 1017 cm-2. Intensity of Si-Nc PL shows the characteristic rising by increasing the temperature up to ?100 K followed by a flattening or a weak reduction up to room temperature. This behaviour reveals a population of radiative states built up by a thermally activated process. Similarly, the measured PL decay-rate is not constant with temperature but shows evidence of a thermal activation. By measuring on different samples the activation energies Ea involved in the temperature dependence of PL intensity and decay time we verify that in all these processes Ea is a decreasing function of implantation dose (i.e., of crystallite size). This result is consistent with models connecting radiative recombination to excitons confined inside Si-Nc, in seeming contrast with the common attribution of PL of non-passivated Si-Nc to the recombination from surface/interface states. To verify the consistency of this statement, we have compared our experimental data with the predictions of quantum confinement theory obtaining an excellent agreement.